1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image by means of electrophotography.
2. Description of the Related Art
Among pieces of image forming apparatus which form an image by means of electrophotography, such as a full-color copier, a printer, and a multifunction machine, one type is equipped with a rotary development unit having four development devices assigned to respective Y (yellow), H (magenta), C (cyan), and K (black) colors. In the image forming apparatus of this type, each of the development devices of the rotary development unit is provided with one development roller.
Rendering the density of an image stable has recently become important for enhancing image quality. Therefore, when a two-component developer formed from toner and a carrier is used, the concentration of toner in the developer (hereinafter simply called “toner concentration” or a “toner mixing ratio”) must be appropriately controlled. Methods for detecting a toner concentration include, in addition to a method for detecting the concentration of toner in a developer agitated in a development device, a method for indirectly ascertaining the concentration of toner in the development device by means of detecting the concentration of toner in the developer carried by a development roller.
An image forming apparatus equipped with a toner concentration sensor (TC sensor) for detecting the concentration of toner on a development roller has become known as a related-art image forming apparatus having a rotary development unit. In such a conventional image forming apparatus, the concentration of toner on the development roller is detected by a toner concentration sensor. On the basis of a result of detection, supply of toner to the development apparatus from the toner cartridge is controlled. Such a system for controlling toner supply using a toner concentration sensor is also called an Auto Toner Concentration (ATC) system.
However, the related-art image forming apparatus adopts a construction for detecting the concentration of toner on a development roller through use of a toner concentration sensor and at a position differing from a position where an electrostatic latent image is actually developed (hereinafter described as a “development position”); e.g., a position displaced from the development position by 90° or 180° in the rotational direction of the rotary development unit. The apparatus suffers the following drawbacks.
Specifically, when toner supply is controlled by means of the ATC system, the concentration of toner on the development roller must first be detected at a position differing from the development position. Subsequently, the development roller must be replenished with toner after having been moved to the development position. Therefore, the image forming apparatus equipped with a rotary development unit cannot control the toner supply in real time.